The Holy Eucharist

1. The matter of the Eucharist is wheaten bread and wine made from the grape mixed with a little water so that the water becomes part of the wine. The water signifies the faithful who are incorporated into Christ. The matter for this sacrament cannot be anything other than wheaten bread and wine from the grape.

2. The form of the Eucharist is the very words of Christ: "This is My Body" and "This is the cup of My Blood, the Blood of the new and everlasting covenant, the mystery of faith, which shall be shed for you and for many, so that sins may be forgiven." These words spoken by the priest in the person of Christ bring into being this sacrament.

3. The minister of the Eucharist is the priest and no one else can consecrate this matter into the Body of Christ.

The effects of the Eucharist

a. Christ becomes physically present
In virtue of the above words, bread is changed into the Body of Christ and wine into His Blood so that Christ is entirely contained under the appearances of bread which remain without a subject and He is entirely contained under the appearances of wine. Moreover, under each part of the consecrated Host and of the consecrated Wine, Christ is totally present even after the separation is made.

b. The Communicant is united with Christ
In the soul of one who worthily receives this sacrament, its second effect is to bring about the union of that man with Christ, as He himself says: "He that eateth my flesh and drinketh my blood abideth in me and I in him." And since man is incorporated with Christ and united to His members through grace, it follows that through this sacrament grace is increased in those who receive it worthily. Therefore, in this sacrament there are three aspects: (1) the sacramentum tantum (that which is the sacrament alone: the species of bread and wine); (2) the res et sacramentum (the true Body of Christ); and (3) the res tantum (the unity of the Mystical Body, the Church) which this sacrament both signifies and causes.

Source:  God's Greatest Gifts